Claims:We Claim:

1. A method for beneficiation of spent magnesia-chrome refractory compositions having 20 to 23% Cr2O3, 53 to 60% magnesia, 10 to 12% iron oxide, 5 to 8% alumina, 1.5 to 3% calcium oxide, 3 to 4% silica and 0.5 to 3% TiO2 to produce non-magnetic product devoid of free iron oxide (FeO, Fe2O3 and Fe3O4) phases comprising:

subjecting the said spent refractory fines to magnetic separation involving high gradient magnetic separator (HGMS) such as to produce said non-magnetic concentrate from the spent MgO-Cr2O3 refractories devoid of free iron oxides.

2. A method for beneficiation of spent magnesia-chrome refractory compositions as claimed in claim 1 wherein said spent magnesia -chrome refractory is sourced from RH degasser in steel plant.

3. A method for beneficiation of spent magnesia-chrome refractory compositions as claimed in anyone of claims 1 or 2 comprising subjecting the spent MgO-Cr2O3 refractory to crushing less than 150 µm size particles to be used as feed for said magnetic separation.

4. A method for beneficiation of spent magnesia-chrome refractory compositions as claimed in anyone of claims 1 to 3 wherein the spent MgO-Cr2O3 refractory is subjected to three-stage HGMS preferably with a finer matrix of 1 mm size.

5. A method for beneficiation of spent magnesia-chrome refractory compositions as claimed in anyone of claims 1 to 4 comprising of three-stage HGMS, wherein the First stage HGMS is carried out involving the magnetic field intensity of 1500 to 2500 Gauss, Second stage is carried out involving the magnetic field intensity of 2500 to 3500 Gaussby feeding the first stage non-magnetic concentrate, and the Third stage is carried out involving the magnetic field intensity of 3500 to 7000 Gaussby feeding the second stage non-magnetic concentrate such as to produce said non-magnetic concentrate with 8 to 9% Fe2O3& 21 to 24% Cr2O3, and tailing with 9 to 11% Fe2O3& 17 to 20% Cr2O3.

6. A method for beneficiation of spent magnesia-chrome refractory compositions as claimed in anyone of claims 1 to 5 wherein pulse rate is maintained in the range of 180 to 250 rpm for all three stages of magnetic separation to achieve maximum recovery.

7. A method for beneficiation of spent magnesia-chrome refractory compositions as claimed in anyone of claims 1 to 6 wherein the slurry solid percent is maintained in the range of 25 to 30% all throughout the process.

8. A method for beneficiation of spent magnesia-chrome refractory compositions as claimed in anyone of claims 1 to 6 wherein the productweight recovery varied from 60 to 78%.

9. Non-magnetic concentrate devoid of free iron oxide (FeO, Fe2O3, Fe3O4) phases comprising 8 to 9% Fe2O3& 21 to 24% Cr2O3, which is sourced form of spent magnesia-chrome refractory compositions.

10. Non-magnetic concentrate as claimed in claim 9 wherein said non-magnetic concentrate comprises:
Cr2O3 :21-24 wt%;
Fe2O3 :8-9 wt %;
MgO :55-62 wt %;
Al2O3 :5-7 wt %;
SiO2 :2-4 wt %;
CaO :1-3 wt %

11. Non-magnetic concentrate as claimed in anyone of claims 9 or 10 can be reused as raw material for manufacturing of magnesia-chromite refractories such as bricks, castable, gunning mass, and the like having high temperature stability up to 1600oC.

Dated the 28th day of October, 2019
Anjan Sen
Of Anjan Sen & Associates
(Applicant’s Agent)
IN/PA-199

, Description:FIELD OF THE INVENTION
The present invention relates to a process for beneficiation of spent magnesia-chromite (MgO-Cr2O3) refractories from RH degasser. More particularly, the present invention is directed to a process for generating a product (non-magnetic concentrate)devoid of free iron oxide (FeO, Fe2O3, Fe3O4) phasesso that it can be reused as a raw material in manufacturing of magnesia-chromite based refractories for high temperature applications. Presently, the spent MgO-Cr2O3 refractory is not recycled and discarded as waste.To beneficiate the spent MgO-Cr2O3 refractory from RH degasser, a three-stage magnetic separation process has been developed according to present invention using high gradient magnetic separator (HGMS) with fine matrix (1 mm)with the magnetic field intensity of 1500to 7000 Gauss,pulse rate in the range of 180 to 250 rpm, and feed size of -150 µm. The overall recovery obtained from the developed process is varying from 60 to 78% weight recovery with 8 to 9% Fe2O3and 21 to 24% Cr2O3. Slurry solid percentage 25 to 30% was maintained during the three-stage HGMS trials. The recovered non-magnetic concentrate after three-stage separation is devoid of any free iron oxide in any form.

BACKGROUND OF THE INVENTION
JSW Steel, Vijayanagar Works at Bellary, Karnataka, has three RH degasser units for secondary refining of value added steels and processes degassing steel grades, interstitial free (IF) grades and electrical steel grades.Said RH degasser units are lined with magnesia-chrome (MgO-Cr2O3) refractory in their inner vessel and snorkel and generate approximately 2200 to 2500 TPA of spent MgO-Cr2O3 refractories which is currently not recycled and discarded as waste.Recycling the spent refractory as a refractory raw material can not only address the natural resource scarcityissue, but also prevent soil pollution and other environmental concerns.

MgO-Cr2O3 refractories, due to their superior thermo-mechanical properties and slag corrosion resistance,find applicationsin steel, nonferrous, and cement industries. The chrome ion in these refractories generally exist in trivalent state, Cr(III) which is non-toxic in nature. However, Cr(III) may get oxidised to toxic hexavalent chromium Cr(VI) under certain service conditions, such as in presence of alkali or alkaline earth oxides at elevated temperatures and thus pose environmental concerns.Even though the use of chromite containing refractories is currently reduced at only 1% of global refractory raw material demand, it is still being continued in some applications such as RH degasser for secondary refining of steel as no suitable alternative is available.After the campaign life of RH degasser, approximately 2200 to 2500 tonnes of spent MgO-Cr2O3 refractories consisting of 53 to 60% MgO, 20 to 23% Cr2O3, 10 to 12% Fe2O3, 5 to 8% alumina, 3 to 4% silica, 1.5 to 3% CaO, and 0.5 to 3% TiO2are generated annually.There has been thus a significant emphasis on recycling of the spent chrome-containing refractories owing to the chromium toxicity.

Korean Patent No. KR 100833025B1by Choet al, dated 25th May 2008, provides a method for manufacturing magnesium chrome clinkers by recycling waste Mg-Cr bricks generated from steelmaking and lime calcination industries through electro-fusion method. The manufacturing method comprises of screening of the refractory bricks, removal of slag and metal, grinding the bricks to obtain a size of less than 20 mm, melting in an arc furnace at a temperature more than 20000C for less than 4h, cooling slowly and grinding to obtain Mg-Cr clinker of more than 100 µm size that contained more than 25 wt% Cr2O3.

European Patent No DE3340849A1dated 23rd Oct1985 by Mortezadescribes a process on recycling of Mag-Chrome and Chrome-Mag refractories from cement industry by moistening in water for 1 to 3 weeks followed by crushing to a size below 50 mm and storing on a concrete surface with periodic spraying of water for a period 3 to 5 weeks to remove any soluble ions or Cr+6. The recovered refractory was comminuted to 0-6 mm size and used to prepare mag-chrome brick for cement industry application.

Chinese Patent No CN107716088A dated 23rdFeb2018by Fen et al claims a method for processing of magnesia-chrome waste by floatation using sulphuric acid as pH value adjusting agent, xanthate and double yellow medicine as collector, terpenic oil as foaming agent, and subsequently heating the tailings in air atmosphere at high temperature to obtain the reusable magnesia-chromite refractory material.

In another study, Hanet al[DOI: 10.1021/acssuschemeng .6b01163] have proposed a method to recycle spent MgO-Cr2O3 bricks generated in copper smelters by floatation process using Na2S, emulsified kerosene and collectors as conditioners.Samples of 80% passing through 74 µm were used for the experiment. The tailingcan be reused as a raw material for new refractory bricks. In this process, they were also able to recover 95 wt% Cu with the concentrate containing 21.4 wt% Cu that can be fed to the smelting process.

Hanewaldet al [Ceramic Engineering and Science Proceedings, 1993,14 (3–4)] have reported about a method to recycle spent chrome refractory from glass industry as a feed material to replace chrome ores in smelting of chromium metal. The method comprised of charging crushed spent refractory directly into electric furnaces where the chrome oxide was reduced by coke or coal at 1600°C, thus bypassing expensive pelletizing and pre-reduction processes.

Simon et al[Materials Transactions, 2003, 44(7), 1251-1254] have developed a process to recycle chromium oxide containing waste refractories by melting with clayin an arc furnace at a temperature more than 2100oC to produce eskolaite-corundum, a Cr2O3-Al2O3 based fused material that could be used as a raw material for new refractories with high corrosion resistance.

John Noga[Ceramic Engineering and Science Proceedings,1994, 15[2], 73-77] has reported a process to produce a recycled MgO-Cr2O3product from glass industry waste by blending magnetically separated MgO-Cr2O3 of less than 20 mesh size andMgO and MgO-Cr2O3 of less than 6 mesh size in desired proportion followed by washing with fresh water and repeated cleaning in modified concrete mixers over a 12h cycle to remove sodium sulphate from glass melting process. The MgO-Cr2O3 is then dried in rotary driers and ball milled to -20 mesh to obtain the final product.

Martin and Petty[Martin E, Petty AV Jr, RI 8489, USBM, 1980] have reported about a method to beneficiate waste MgO-Cr2O3bricks from argon-oxygen decarburization (AOD) vessels and electric-arc furnaces (EAF) and subsequently recycle the beneficiated aggregates to produce magnesia-chrome brick. The beneficiation process consisted of crushing and screeningof the waste refractory, and removing any steel penetration or steel casings from the chrome refractory concentrate by magnetic separation.They concluded that crushing to a size less than 6 mm wasadequate to separate non-magnetic concentrate having a chemical composition comparable to original MgO-Cr2O3 aggregate.

Martin and Petty [U.S. Department of the Interior, Bureau of Mines, RI 8589, 1981, 1-18] have also developed a process to recycle MgO-Cr2O3 bricks from copper-smelting furnaces by grinding the spent bricks to -65 µm size to liberate themetallic copper and beneficiating in a low intensity magnetic separator followed by floatation. Due to very fine size, they also developed another process by leaching coarser grains of -6 mesh size using ammonia-ammoniumcarbonate to yield clean refractory aggregate. Finally, the beneficiated refractories were heatedabove 1700°C to eliminate the glassy impurities.

During processing of IF and electrical steel grades in RH vessel, oxygen blowing is done to adjust the steel quality. Lot of iron oxides get generated in the process which can diffuse into the refractory easily and remain either as free iron oxides or form solid solution with MgO and chromite spinel grains in the refractory layer. Presence of free iron oxide in a refractory is undesirable as it tends to form low melting phases at high temperature.

The present invention provides a process of beneficiating spentMgO-Cr2O3 refractoriesfrom RH degasser by using a three-stage magnetic separation process to effectively separate free iron oxide (FeO, Fe2O3, Fe3O4) phases which can berecycled as raw material in manufacturing of magnesia-chromite based refractories.

OBJECTS OF THE INVENTION

The basic object of the present invention is directed to provide a process for beneficiation of spent MgO-Cr2O3refractoriesobtained from RH degasser,through which interstitial free, electrical and degassing steel grades are processed, to generate a non-magnetic concentrate product devoid of free iron oxide (FeO, Fe2O3, Fe3O4) phases to be reused as a raw material in manufacturing of magnesia-chromite based refractories for high temperature applications.

A further object of the present invention is directed to provide a process for beneficiation of spent MgO-Cr2O3refractories obtained from RH degasser wherein maximum recoverycan be achieved involving three-stage magnetic separation process using high gradient magnetic separator (HGMS).

Another object of the present invention is directed to provide a process for beneficiation of spent MgO-Cr2O3 refractories obtained from RH degasser wherein the spent refractories crushed to -150 µm size can be used as the feed material.

A still further object of the present invention is directed to provide a process for beneficiation of spent MgO-Cr2O3refractories obtained from RH degasser wherein selective magnetic field strength is applied to each stage of said three-stage magnetic separation process to significantly improve the grade and weight recovery.

Another object of the present invention is directed to provide a process for beneficiation of spent MgO-Cr2O3refractories obtained from RH degasser wherein selective magnetic field pulse rate is applied to each stage of said three-stage magnetic separation process to significantly improve the grade andweight recovery.

Yet another object of the present invention is directed to provide a process for beneficiation of spent MgO-Cr2O3refractories wherein the first stage magnetic tailing is discarded and non-magnetic concentrate is used as feed to second stage and subsequently to third stage magnetic separation to achieve the desired product.

Another object of the present invention is directed to provide a processfor beneficiation of spent MgO-Cr2O3refractories obtained from RH degasser wherein the product obtained is devoid offree iron oxide (FeO, Fe2O3, Fe3O4) phases.

Yet another object of the present invention is to use the obtained product as raw material to manufacture magnesia-chromite refractories such as bricks, gunning mass, castables, etc. for high temperature applications (up to 1600oC).

SUMMARY OF THE INVENTION

The basic aspect of the present invention is directed to a method for beneficiation of spent magnesia-chrome refractory compositions having 20 to 23% Cr2O3, 53 to 60% magnesia, 10 to 12% iron oxide, 5 to 8% alumina, 1.5 to 3% calcium oxide, 3 to 4% silica and 0.5 to 3% TiO2 to produce non-magnetic product devoid of free iron oxide (FeO, Fe2O3 and Fe3O4) phases comprising:
subjecting the said spent refractory fines to magnetic separation involving high gradient magnetic separator (HGMS) such as to produce said non-magnetic concentrate from the spent MgO-Cr2O3 refractories devoid of free iron oxides.

A further aspect of the present invention is directed to a method for beneficiation of spent magnesia-chrome refractory compositions wherein said spent magnesia-chrome refractory is sourced from RH degasser in steel plant.

A still further aspect of the present invention is directed to a method for beneficiation of spent magnesia-chrome refractory compositions comprising subjecting the spent MgO-Cr2O3 refractory to crushing less than 150 µm size particles to be used as feed for said magnetic separation.

Another aspect of the present invention is directed to a method for beneficiation of spent magnesia-chrome refractory compositions wherein the spent MgO-Cr2O3 refractory is subjected to three-stage HGMS preferably with a finer matrix of 1 mm size.

Yet another aspect of the present invention is directed to a method for beneficiation of spent magnesia-chrome refractory compositions comprising of three-stage HGMS, wherein the First stage HGMS is carried out involving the magnetic field intensity of 1500 to 2500 Gauss, Second stage is carried out involving the magnetic field intensity of 2500 to 3500 Gaussby feeding the first stage non-magnetic concentrate, and the Third stage is carried out involving the magnetic field intensity of 3500 to 7000 Gaussby feeding the second stage non-magnetic concentrate such as to produce said non-magnetic concentrate with 8 to 9% Fe2O3& 21 to 24% Cr2O3, and tailing with 9 to 11% Fe2O3& 17 to 20% Cr2O3.

A further aspect of the present invention is directed to a method for beneficiation of spent magnesia-chrome refractory compositions wherein pulse rate is maintained in the range of 180 to 250 rpm for all three stages of magnetic separation to achieve maximum recovery.

A still further aspect of the present invention is directed to a method for beneficiation of spent magnesia-chrome refractory compositions wherein the slurry solid percent is maintained in the range of 25 to 30% all throughout the process.

A still further aspect of the present invention is directed to amethod for beneficiation of spent magnesia-chrome refractory compositions wherein the product weight recovery varied from 60 to 78%.

A still further aspect of the present invention is directed to a Non-magnetic concentrate devoid of free iron oxide (FeO, Fe2O3, Fe3O4) phases comprising 8 to 9% Fe2O3& 21 to 24% Cr2O3, which is sourced form of spent magnesia-chrome refractory compositions.

A still further aspect of the present invention is directed to a Non-magnetic concentrate wherein said non -magnetic concentrate comprises:
Cr2O3 :21-24 wt%;
Fe2O3 :8-9 wt %;
MgO :55-62 wt %;
Al2O3 :5-7 wt %;
SiO2 :2-4 wt %;
CaO :1-3 wt %

A still further aspect of the present invention is directed to a Non-magnetic concentrate comprising of bricks, castable, gunning mass, and the like having high temperature stability of up to 1600oC.

In the above process, the pulse rate of the magnetic field in all the three stages was kept in the range of 180 to 250 rpm, preferably 220 rpm to maximize the overall recovery and grade.

In this process, the spent refractories are crushed to -150 µm particle size to be used as the feed.

Yet another aspect of the present invention is directed to said process of three stage magnetic separation wherein the final product comprises of 8 to 9% Fe2O3 and 21 to 24% Cr2O3 having a melting point more than 1600 oC.

The present invention is described in greater details with reference to following accompanying non limiting illustrative drawings and examples.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

Figure 1: Shows Mapping of spent MgO-Cr2O3 refractory.
Figure 2: illustrates the process flow sheetfor Three-stage HGMS according to present invention for beneficiation of spent MgO-Cr2O3 refractories from RH degasser.
Figure 3: shows XRD of spent MgO-Cr2O3, and magnetic tailing and non-magnetic concentrate after three stage HGMS separation.
Figure 4: shows pictorially Melting temperature measured of non-magnetic concentrate product after three-stage HGMS separation.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO ACCOMPANYING DRAWINGS

The present invention relates to a process for beneficiating spent MgO-Cr2O3 refractories from RH degasser to generate a product devoid free iron oxides that can be reused as a raw material in manufacturing of magnesia-chromite refractories for high temperature applications. The process according to present invention directs at producing a non-magnetic concentrate product through three-stage magnetic separation using high gradient magnetic separator (HGMS) with fine matrix (1 mm) having magnetic field intensity in the range of 1500 to 7000 Gauss with a feed size of -150 µm and pulse rate of 180 to 250 rpm. The overall recovery obtained from the developed process is varying from 60 to 78% weight recovery with 8 to 9% Fe2O3 for input feed Fe2O3 10 to 12% and 21 to 24% Cr2O3 for input feed Cr2O3 20 to 23%. Slurry solid percentage 25 to 30% was maintained during the two-stage HGMS trials.

The MgO-Cr2O3 refractories used in RH degasser are re-bonded or semi re-bonded ones having nominal composition as shown in Table 1.Iron oxide generated during refining of steel through RH degasser can easily penetrate the refractory, remain as an impurity in the pores or form solid solution with periclase and/or chromite spinel by migrating into their crystal structure. Iron oxide, if present as an impurity, can reduce the melting temperature of the refractory and affect its high temperature performance. Area scanning mapping of the spent refractory (Figure 1) shows the periclase (MgO) and iron bearing chromite spinel grains [(Fe,Mg)O•(Cr,Al,Fe)2O3], also known as complex spinel,to be directly bonded. The iron oxide rich phases were also present at the grain boundaries.
Table 1: Chemical analysis of MgO-Cr2O3 bricks used in RH degasser (wt%)

MgO Cr2O3 Al2O3 Fe2O3
58 - 70 18 - 28 5-8 6-10

To beneficiate the spent MgO-Cr2O3 refractories from RH degasser, a three-stage magnetic separation process using HGMS with fine matrix (1 mm) has been employed to capture the maximum non-magnetic materials. The flow diagram of the process is shown in Figure 2. The spent MgO-Cr2O3 refractories, crushed to -150 µm size,weretreated in HGMS (Stage-I) by maintaining the magnetic field intensity 1500 to 2500Gauss. The non-magnetic concentrate was fed to HGMS (Stage-II) with a magnetic field intensity of 2500 to 3500 Gauss. The second stage non-magnetic concentrate was fed to HGMS (Stage-III) maintaining a magnetic field intensity of 3500 to 7000 Gauss. The non-magnetic concentrate obtained after Stage-III HGMS was the final product. In all three stages, pulse rate and solid percent in slurry were maintained at 180 to 250 rpm and 25 to 30% respectively.Theproduct gradeand weight recovery are shown in Table 2.

Table 2:Beneficiated spent refractory product and tailing analysis (wt%)

Wt% Cr2O3 MgO Fe2O3 Al2O3 SiO2 CaO
Feed 100 20-23 53-60 10-12 5-8 3-4 1.5-3
Non-Magnetic Concentrate 60-78 21-24 55-62 8-9 5-7 2-4 1-3
Magnetic Tailing 22-40 17-20 51-53 9-11 3-5 1-3 0.5-2

The process developed for beneficiating the spent MgO-Cr2O3 refractories has the following novel features:

1. Selective separation of free iron oxide (FeO, Fe2O3, Fe3O4) phases present in spent MgO-Cr2O3 refractory fines using high gradient magnetic separation (HGMS).

2. Design of the beneficiation circuit for processing ofspent MgO-Cr2O3 refractory fines through magnetic separation to maximize theconcentrate recovery.

Phase analysis of the spent MgO-Cr2O3 refractory, tailing and concentrate after three-stage HGMS is illustrated in Figure 3. The spent refractory was found to have substantial amount of hematite (Fe2O3) and magnetite (Fe3O4). After beneficiation, the tailing contains Fe2O3 and Fe3O4as minor phases whereas these phases were not found in the concentrate.

The melting temperature of the concentrate after three-stage HGMS is shown in Figure 4. The recovered material was found to have a melting temperature more than 1600oC. Thus, the recovered material can be used as a raw material in processing of different MgO-Cr2O3 based refractories such as bricks, castable, gunning mass, etc. for various high temperature applications (up to 1600oC).

The present invention is further substantiated with the example given below:

Example 1: Spent MgO-Cr2O3 refractory obtained from lower vessel of RH degasser having 53.02% MgO, 21.66% Cr2O3, 10.76% Fe2O3, 5.51% Al2O3, 1.95% CaO and 3.54% SiO2.

A process for beneficiation of the said spent refractory comprising steps of crushing and grinding to -150 µm size and subjecting to a three-stage HGMS separation by applying 2000 Gauss, 3000 Gauss and 4000 Gauss magnetic fields for rougher, scavenger and cleaner stages respectively with a pulse rate of 250.

Recovery of non-magnetic concentrate: 66%. The chemical indicators: Fe2O3: 9.01%, Cr2O3: 23.76%. Melting temperature of non-magnetic concentrate: > 1600oC.
As described above, the present invention has advantages in that the spent MgO-Cr2O3 refractories from RH degasser, which was currently not recycled and disposed as a waste in the inventors’ plant, can be beneficiated and the product obtained therefrom by the present invention can be recycled as a raw material for manufacturing magnesia-chromite refractories such as bricks, gunning mass, castable, etc. for high temperature applications (up to 1600oC).It is thus possible by way of the present invention to provide a beneficiated MgO-Cr2O3 spent refractory with 60-78% recovery and devoid of any free iron oxide,thereby recovering a valuable refractory material from the waste.